NONEQUILIBRIUM AIR CLATHRATE HYDRATES IN ANTARCTIC ICE - A PALEOPIEZOMETER FOR POLAR ICE CAPS

''Craigite,'' the mixed-air clathrate hydrate found in polar ice caps below the depth of air-bubble stability, is a clathrate mixed crystal of approximate composition (N2O2).6H2O. Recent observations on the Byr d Station Antarctic core show that the air hydrate is present at a dep th of 727 m, well above the predicted depth for the onset of hydrate s tability. We propose that the air hydrate occurs some 100 m above the equilibrium phase boundary at Byrd Station because of ''piezometry''-i .e., that the anomalous depth of hydrate occurrence is a relic of a pr evious greater equilibrium depth along the flow trajectory, followed b y vertical advection of ice through the local phase-boundary depth. Fl owline trajectories in the ice based on numerical models show that the required vertical displacement does indeed occur just upstream of Byr d Station. Air-hydrate piezometry can thus be used as a general parame ter to study the details of ice flow in polar ice caps and the metasta ble persistence of the clathrate phase in regions of upwelling blue ic e.